The present invention relates to an image sensor used in image scanners, facsimile machines, and the like, and more particularly to an image sensor capable of producing stabilized image signals, and therefore ensuring an exact image read.
In a conventional image sensor, particularly a contact type image sensor, the length of the image sensor is equal to the width of the original document placed thereon for an image reading. The image on the original is projected on the image sensor at the ratio of 1:1. The image sensor outputs the image projected thereon in the form of electrical signals. The image sensor receives the image projected thereon by a great number of photo sensing elements. In other words, it fetches the image information of the received image in the form of a great number of pixels.
In a TFT-drive image sensor typically known as a contact image sensor, the photo sensing elements, which are grouped into a plural number of blocks, generate charge when they receiving the image information. The charges from the photo sensing elements are primarily stored in stray capacitors in every block by using switching elements or thin film transistors (TFTs). Then, the charges are sequentially read out of the stray capacitors in the form of electrical signals at a rate of several hundreds kHz to several hundreds MHz. With use of the TFTs, a single drive IC is provided for the image read operation. Accordingly, the TFT-drive image sensor requires fewer drive ICs.
Turning to FIG. 2, there is shown an equivalent circuit of a conventional TFT-drive image sensor. As shown, the TFT-drive image sensor is made up of a sensor array 11, a charge transfer portion 12, and a matrix wire portion 13.
The sensor array 11 consists of a linear array consisting of a plural number of photo sensing elements 11". The length of the linear array is approximately equal to the width of an original document.
The charge transfer portion 12 includes first thin film transistors TTi,j (i=1 to N and j=1 to N) provided for the photo sensing elements 11" in one-to-one correspondence, and second thin film transistor TRi,j (i=1 to N and j=1 to N) also provided for the photo sensing elements 11" in the same way.
The matrix wire portion 13 consists of multilayered wires arranged in a matrix fashion.
The first thin film transistors TTi,j are used for charge transfer. The second thin film transistors TRi,j remove the charge left in the photo sensing elements 11", thereby resetting the photo sensing elements.
The sensor array 11 is divided into of an N number of blocks each consisting of n number of photo sensing elements 11". The photo sensing elements 11" may be equivalently expressed in terms of photo diode Pi,j (i=1 to N, and j=1 to n).
As shown, the photo sensing element 11" is connected to the drain electrode of the first thin film transistor TTi,j, and also to the drain electrode of the second thin film transistor TRi,j. The source electrode of the second thin film transistor TRi,j is earthed. The source electrodes of the first thin film transistors TTi,j are connected through the matrix wire portion 13 to an n number of common signal lines 14, which are connected to a drive IC 15.
The gate electrodes of the first thin film transistors TTi,j and those of the second thin film transistors TRi,j are connected to a gate pulse generator 16, so that those transistors are turned on every block.
The light charges generated in the photo sensing elements 11" are stored in the stray capacitors of the photo sensing elements and the train-gate overlap capacitors of the thin film transistors, for a preset period of time. Then, the charges are successively transferred to the wire capacitors CLi (i=1 to n) of the matrix wire portion 13 by the first thin film transistor TTi,j as switching elements.
In operation, a gate pulse .phi.GT1 is transferred from the gate pulse generator 16 through gate lines GTi (I=1 to n) to the first thin film transistors TTi,1 to TTi,n in the first block. These transistors are turned on, and the charges generated in the photo sensing elements 11" in the first block are transferred and stored in the wire capacitors CLi. As the result of the storage of the charges, the potential of the grouped signal lines 14 is varied. The varied potential values are sequentially output from an output line 17 by successively turning on analog switches SWi (i=1 to n) (not shown) in the drive IC 15.
Further, the gate pulse generator 16 transfers a gate pulse .phi.GR1 through a gate line GRi (i=1 to n) to the second thin film transistors TR1,2 to TR1,n in the first block, which are then turned on. As a result, the charges left in the stray capacitors of the photo sensing elements and the drain-gate overlap capacitors of the thin film transistors are removed.
Gate pulses .phi.GT2 to .phi.GTn turn on the first thin film transistors TT2,1 to TT2,n TTN,1 to TTN,n in the second to Nth blocks, thereby to transfer the charges of the photo sensing elements of every block. Gate pulses .phi.GR2 to .phi.GRn turn on the second thin film transistors TR2,1 to TR2,n TRN,1 to TRN,n in the second to Nth blocks, thereby removing the charges left in the photo sensing elements of every block. The varied potentials in the grouped signal lines 14, caused by the charges transferred thereto, are successively output. In this way, image signals of one line in the main scan direction on the original are obtained. Thereafter, the image sensor is returned to the home position by a suitable mechanical means, for example, roller means (not shown), and then the above sequence of operations is repeated until the image signals of the entire image on the original are obtained (see Published Unexamined Japanese Patent Application No. Hei. 2-265362).
The structure construction of the thus electrically arranged TFT-drive image sensor, which includes the photo sensing elements, and the first and second thin film transistors, will be described with reference to FIG. 2.
A wire 30a from a photo sensing element 11" is connected to a drain electrode 41 of the first and second thin film transistors (TFTs) TT and TR. The drain electrode 41 is used commonly for the first and second thin film transistors in order to reduce the area of the image sensor, which extends in the vertical scan direction. In the first thin film transistor TT, a source electrode 42T extends to the matrix wire portion 13 where it is connected to grouped signal lines 14. The drain electrode 41 and the source electrode 42T partially overlap with the gate electrode 25T.
In the second thin film transistor TR, a source electrode 42R is connected to a light-shield layer 30 for shielding the first and second thin film transistors TT and TR from light. The light-shield layer 30 is kept at constant voltage level, e.g., ground potential. The drain electrode 41 and the source electrode 42R partially overlap with the gate electrode 25R.
The method of reading image information on the original by the TFT-drive image sensor thus constructed will be described with reference to FIG. 3 showing a timing chart.
A variation of the potential at the first thin film transistor TT is illustrated in FIG. 3(a). As shown, in a dark state, the potential gradually increases with time during the light charge storage. In a bright state, it increases relatively greatly. When the thin film transistor TT is turned on, its conduction increases steeply by a voltage caused by a feed-through voltage (referred to as a feed-through-caused-voltage).
As previously, in the transistor, the source and drain electrodes partially overlap with the gate electrode. Accordingly, the transistor inevitably contains overlap capacitance. When a large gate pulse is applied to the gate electrode, the potential is instantaneously increased by action of the overlap capacitance. When the gate pulse is removed, the potential is instantaneously decreased. The instantaneous increase or decrease of voltage is the feed-through voltage. The amplitude of the feed-through voltage is expressed by the product of multiplying the difference between the gate voltages of the on and off states of the gate pulse, viz., a gate voltage swing, by a ratio of the overlap capacitance to the capacitor connected to the source or drain electrode.
The charge is transferred from the drain electrode of the first thin film transistor TT so that the potential at the drain electrode is in equilibrium with the potential at the source electrode. The potential of the drain electrode decreases, to turn off the first thin film transistor TT. When the transistor is turned off, the potential at the drain electrode drops steeply by the feed-through-caused-voltage. The potential still present on the drain electrode after the drop in potential, is the potential caused by a residual charge. The second thin film transistor TR is turned on. Then, the potential on the drain electrode abruptly increases by the fed-through-caused-voltage. Then, the charge is transferred till it reaches the ground potential, and the potential decreases. When the second thin film transistor TR is turned off, the potential abruptly decreases by the feed-through-caused-voltage. Another charge storage starts at the decreased potential.
As shown in FIG. 3(b), at the source electrode of the first thin film transistor TT the potential is constant during the period of the light charge storage. When the first thin film transistor TT for charge transfer is turned on, the potential thereat abruptly increases by the feed-through-caused-voltage. At this time, the charge is transferred from the source electrode of the first thin film transistor TT so that the potential at the source electrode is in equilibrium with the potential at the drain electrode. And the potential at the source electrode increases to turn off the first thin film transistor TT. When the first thin film transistor TT is turned off, the potential at the source is abruptly decreased by the feed-through-caused-voltage. That potential is sensed and corresponds to the quantity of the transferred charge.
The MOS transistor is turned on by a negative voltage, and the potential deceases by the feed-through-caused-voltage. The charge is transferred till the potential reaches the ground potential, so that the potential decreases. In this case, the reset overlaps with the voltage drop by the feed-through-caused-voltage since the on-resistance of the MOS transistor is smaller than that of the thin film transistor. The MOS transistor is turned off, and the potential sharply increases by the feed-through-caused-voltage. This potential is the potential in the initial stage, and is sensed as a reference potential. The difference between the potential previously sensed an the reference potential is the sensor output signal.
In the conventional image sensor, if a great offset is created in the sensor output signal by an alignment displacement, which is caused in the fabricating stage, the light charge start potential (the potential at which the light charge storage starts) at the drain electrode of the first or charge-transfer thin film transistor TT is greatly shifted to the positive or the negative side. The output potential at the source electrode of the first thin film transistor TT is greatly shifted to the positive or the negative side. Accordingly, the output signal in the dark state does not approximate to 0 (zero). The range of the amplifier for amplifying the sensor output signal is large, so that the output signal of the amplifier greatly varies, viz., the sensor output is instable.
This offset problem will be described in detail.
In the case of the image sensor constructed as shown in FIG. 2, if the overlapping areas of the source and drain electrodes with the gate electrode in the first and second thin film transistors TT and TR are charged by the alignment displacement, the overlap capacitance of the overlapping areas change. Further, the feed-through-caused-voltage in the first thin film transistor TT is different from that in the second thin film transistor TR. As a consequence, a great positive or negative going offset is created.
In the image sensor of FIG. 2, a chromium (Cr1) layer is used for forming the gate electrode 25T of the first thin film transistor TT. The gate electrode 25R of a second thin film transistor TR is also formed with the chromium layer. A chromium layer (Cr2) is used for forming the drain and the source electrodes of the transistors TT and TR.
Let us consider a case where the areas of the drain and the source electrodes 41, and 42T and 42R, overlapping with the gate electrodes 25T and 25R, are displaced to the left from the gate electrodes. In the first thin film transistor TT, the overlapping area of the gate electrode 25T with the drain electrode 41 increases, while the overlapping area of the gate electrode 25T with the source electrode 42T decreases. In the second thin film transistor TR, the overlapping area of the gate electrode 25R with the drain electrode 41 decreases while the overlapping area of the gate electrode 25R with the source electrode 42R increases.
In this case, the feed-through-caused-voltage at the drain electrode of each of those thin film transistors, as shown in FIG. 3(c), is large in the first thin film transistor TT in which the area overlapping with the drain electrode 41 is large. It is small in the second thin film transistor TR in which the area overlapping with the drain electrode 41 is small. Accordingly, the potential after the resetting operation by the second thin film transistor TR does not become small in comparison with that shown in FIG. 3(a). This results in an offset going greatly positive with respect to the light charge storage start potential at the drain electrode. As shown in FIG. 3(d), the output potential at the source electrode of the first thin film transistor TT has a large positive going offset. In other words, the dark output signal in the dark state is large.
In another case in FIG. 2 where the areas of the drain and the source electrodes 41, and 42T and 42R, overlapping with gate electrodes 25T and 25R, are displaced from the gate electrodes to the right, in the first thin film transistor TT, the overlapping area of the grate electrode 25T and the drain electrode 41 increases, while the overlapping area of the gate electrode 25T and the source electrode 42T decreases. In the second thin film transistor TR, the overlapping area of the gate electrode 25R and the drain electrode 41 decreases while the overlapping area of the gate electrode 25R and the source electrode 42R increases. Therefore, the offsets of the alignment described above cause an instable output.